1. Field of the Invention
The present invention relates to a method and a device for acoustically detecting cracks in metal pieces having a same shape and made of a same metal, for example hardened steel grinder balls, which detecting method and device may be used respectively in a method and a device for automatically sorting out the cracked metal pieces.
2. Description of the Prior Art
Regarding specifically hardened steel grinder balls, it has been reported by manufacturers of such hardened steel grinder balls for ball mills that a non-negligeable percentage of the balls produced exibit deep cracks, and therefore tend to rupture under operating conditions inside the ball mill. This problem is familiar to ball mill operators who often have shut the mill down to remove the broken ball pieces from inside the mill to ensure smooth operating conditions of the mill and to prevent excessive wear of the inner sleeve of such a ball mill. This unpleasant, time consuming job obviously raises the production costs.
The present method used to limit the number of cracked grinder balls introduced in the ball mill is to sort them out manually. According to this present method, the cracks in the grinder balls can be detected both visually and acoustically. The acoustic detection of cracks can be carried out by knocking together two grinder balls. If at least one of these two balls knocked together is cracked, a characteristic high pitch sound is generated, which high pitch sound can be recognized by the operator.
However, such a manual inspection is very costly if compared with the low profit margin obtained for this kind of crude product, namely hardened steel grinder balls. Inspection of each and every grinder ball would be therefore prohibitive. At the present time, only samples retained from each batch of produced grinder balls are inspected in order to give a statistical estimate of the percentage of cracked balls in each particular batch of produced grinder balls. This limited sorting is expensive to the manufacturers, gives only an estimated percentage of cracked balls and does not provide any real time control of quality on the line of production of the grinder balls.
In U.S. Pat. No. 3,284,192 granted to GENERAL MOTORS CORPORATION on Nov. 8, 1966, it is described a device to evaluate mechanical properties of the material a cast metal piece. This evaluation of the mechanical properties of the cast piece is based on an accurate acoustic measurement of the vibration resonance frequency of this cast piece. In order to measure this resonance frequency, the cast piece has to be mounted on a sponge rubber base and thereafter to be stroken by a mallet. It is evident that the design of this prior art device is specifically adapted to evaluate mechanical properties of the material of a cast piece by measuring the vibration resonance frequency of this piece. Moreover, it does not provide a very simple method of detecting cracks in metal pieces and it requires a mounting of the tested pieces on the rubber base.
U.S. Pat. No. 4,122,723 granted to FIAT SOCIETA PER AZIONI on Oct. 31, 1978, proposes a device for testing finished part using transducers and accelerometers to induce the measure vibrations in the tested part, which has to be rigidly mounted on a mechanical assembly. Again, the measurement carried out by this testing device is relatively complex and the tested piece must be closely coupled with the transducers and accelerometers of the device.
U.S. Pat. No. 4,020,678 dated May 3, 1977, describes a device for testing the teeth of a gear. This prior art device comprises an electromagnet for generating a magnetic field pulse to induce a vibration to each gear tooth and a circuit to detect this vibration and to analyze the amplitude and frequency of the vibration of the gear tooth. The electromagnet must be coupled in sequence with each gear tooth and therefore such a device does not allow a relatively high rate (metal pieces per time unit) of testing operations. Moreover, the measurement carried out by this testing device is relatively complicated due to the required analysis of amplitude and frequency of the vibration of the gear tooth.